As global temperatures rise, the impact on health becomes increasingly apparent, especially for vulnerable seniors. This study analyzed the additional financial burden caused by heatwaves on outpatient healthcare spending among China’s older people, using data from the China Health and Retirement Longitudinal Study (CHARLS) covering 126 cities between 2011 and 2018. Applying generalized estimating equations (GEE) models and incorporating local weather data, we examined the data from 22 023 questionnaires. The findings show that each heatwave in the past 30 d increased individual outpatient costs by 2.29 CNY (95% CI: 0.31–4.27), affecting individuals with chronic diseases, those without air conditioning, and those in rural areas more severely. Naps proved effective in mitigating heatwave effects. Projections suggest that in the summer of 2050, under the SSP5-8.5 scenario, additional healthcare expenses attributable to rising temperatures might reach 29.14 billion CNY (95% CI: 3.94–54.33). These findings highlight the urgent need for proactive heat and health strategies in China and support naps as a protective measure against heat-induced health risks for older people.